Homeotropic alignment of a liquid crystal occurs when the long axes of the molecules of the liquid crystal phase are, on average, essentially normal to the substrate upon which they form a thin film. Very few materials spontaneously align in this way and thus some kind of aligning agent is required to produce this desired alignment. Orthogonal liquid crystal phases like nematic liquid crystal phases and orthogonal smectic phases (e.g. smectic A and smectic B) can be aligned in this way. An Overview about orientation technologies for liquid crystals is given e.g. in J. Cognard, Molecular Crystals Liquid Crystals 78 Supplement 1 (1981) pp 1–77 and in J. A. Castellano MCLC, 94 (1983) pp 3341.
Conventional aligning agents for liquid crystals are designed to be effective on glass substrates, some examples of such conventional organic aligning agents are lecithin, trichloro- and trimethoxypropyl silane, hexadecyl trimethyl ammonium halides and alkyl carboxylato monochromium salts. In each case a very small amount (of typically less than 1%) of the active component is dissolved in a suitable volatile solvent and spread onto the substrate e.g. by spin coating or other well known methods, when the solvent has been evaporated, a thin film of the organic aligning agent remains on the substrate. These materials are characterized by having a polar end group which is assumed to be attracted to the polar glass surface and a long alkyl chain which orientates normal to the glass surface. In some cases a chemical reaction is essential i.e. in the case of chlorosilanes which react with hydroxyl groups on the glass surface. Spontaneous homeotropic alignment of liquid crystals occurs on such surfaces.
Inorganic aligning agents can also be used. For example it is well known that evaporation at normal incidence of SiO2 or MgF2 onto a substrate of glass can be used to produce homeotropic alignment of liquid crystals.
This patent application, however, relates to the alignment of orthogonal liquid crystal phases especially of nematic and of smectic A liquid crystals on a plastic substrate. The plastic substrates are preferably flexible substrates and may be plastic films or also anisotropic liquid crystal polymer films. It was found that the conventional aligning agents are not effective for the alignment of liquid crystals on such plastic substrates. Typically and almost generally either no alignment or only very poor alignment is exhibited. It is suspected that the surface of the plastic substrate has much less affinity for the polar end groups of the aligning agents.
In contrast to the conventionally used organic aligning against, inorganic aligning agents such as typically used on glass surfaces e.g. SiO2 have been found to provide good homeotropic alignment when sputter coated or evaporated onto plastic substrates. It was found, however, that TiO2-coated substrates do not align so well and are more difficult to wet with the usual coating solvents. One of the problems with these types of coatings which is common to SiO2, TiO2 and others, is that they tend to have a significant surface roughness which prevents the polymer film being released from the aligning layer after polymerisation. This could be attributed to the well known fact that adhesion is very much enhanced by using microscopically rough surfaces compared to smoother surfaces. As these coatings are usually deposited onto a high temperature stable substrate such as polyethylene terephtalate (PET) which, due to the stretching processes used in its manufacture, is birefringent the aligned polymer film must be removed from the substrate before the optical properties of the coated polymer itself are accessible.